Modulation Schemes

S. S. Sastry

The point to modulation is to take a message bearing signal
and superimpose it upon a carrier signal for transmission. For
ease of transmission carrier signals are generally high frequency for
severable reasons:

So that they may be simultaneously transmitted without
interference from other signals

So as to enable the construction of small antennas (a fraction,
usually a quarter
of the wavelength)

So as to be able to multiplex that is to combine multiple
signals for transmission at the same time.

For example AM radio is 550-1600 KHz, FM radio is 88 MHz-108 MHz, TV
is 52-88 MHz (channels 1-6), 174-216 MHz (channels 7-12) and
470-900 MHz (UHF),
microwave and satellite signals are of the order of
several GHz and infra red
fiber optic signals are of the order of 200-300 THz.